1. Field of the Invention
The present invention relates to a technology by which a volatile organic compound can be recovered effectively from a mixed gas containing the organic compound in the state of gas to avoid the compound from being diffused in atmospheric air.
2. Discussion of the Background
Volatile hydrocarbons are diffused from storage tanks in atmospheric air when atmospheric temperature rises. Also, the volatile hydrocarbons are diffused in atmospheric air when they are flowed into storage tanks or when they are filled in tank lorries from storage tanks. Diffused hydrocarbons are said to form substances causing "photochemical smog". Accordingly, regulations have been carried out in Japan and other countries to control the concentration of the hydrocarbons in an exhaust or flue gas, and more severe regulations have lately been issued in several countries, the United States being a leading country.
For instance, a standard hydrocarbon concentration in the exhaust or flue gases at outlets is established under a prefectural regulation in Nagoya, Japan, which is the severest regulation in the past and by which the hydrocarbon concentration at the outlets is prescribed to be lower than 5% by volume (corresponding to about 80% by volume in terms of the recovery (as referred hereinafter) of hydrocarbon from a gas having a hydrocarbon concentration of 21% by volume at inlets). Under EPA in the United States, however, such harsh standards have been issuing that the discharge of VOC (volatile organic compound) shall be less than 35 mg/l gasoline for bulk gasoline terminals newly constructed or expanded after Dec. 17, 1980 and that recovery at vapor recovery systems shall be higher than 95% by weight for facilities storing petroleum products having a vapor pressure of 78 to 570 mmHg and newly constructed or expanded after May 19, 1978.
Several methods can be taken into consideration for recovering a hydrocarbon from a vapor. For instance, there have been proposed methods in which the hydrocarbon is separated from a vapor containing the hydrocarbon by adsorption with porous adsorbents such as activated carbons or separated by low temperature processing.
In the adsorption method, however, there is a danger of firing since a large amount of the heat of adsorption will be generated. In the low temperature processing method, the hydrocarbon vapor must be cooled down to a temperature lower than -35.degree. C. in order to increase the recovery percentage since the vapor has a very low liquefying temperature and thus this method is economically disadvantageous.
On the other hand, a method is known for separating a hydrocarbon from a hydrocarbon vapor safely and effectively wherein the vapor is washed with a liquid to absorb at an ambient temperature under an atmospheric pressure, the hydrocarbon absorbed in the liquid is separated to recover, and then the liquid is recycled to the washing.
Suitable liquids used for the absorption are ones which are insoluble in water, have a strong absorption power to hydrocarbon gas, and having such a low vapor pressure such that the liquids will not be lost when hydrocarbon is separated for recovery. Examples of the liquids comprise, as a major component, at least one compound selected from the group consisting of phthalic acid esters, silicic acid esters, phosphoric acid esters, fatty acid esters, alkylbenzene, alkylnaphthalene, and .alpha.-olefins. The liquids may additionally contain less than 75% by weight of a refined mineral oil having a viscosity of 5 to 20 cst at a temperature of 37.8.degree. C., a boiling point of 250.degree. to 450.degree. C., and an average molecular weight of 200 to 350.
According to the absorption method, the hydrocarbon concentration at the outlets can be controlled lower than 5% by volume by using a system composed of an absorption column, desorption column, recovery column, and vacuum pump. However, the absorption method has a defect that a severer standard regulated under, for example, the EPA in the United States can not be cleared.
An improved method for recovering a volatile organic compound from a gas by an absorption method has been proposed in U.S. Pat. No. 4,102,983 wherein a liquid mixture of an ester of phosphoric acid, silicic acid, or fatty acid with a mineral oil is used, an absorption liquid containing a volatile organic compound dissolved therein is subjected to flashing under a reduced pressure in a desorption column, and the liquid subjected to the flashing is recycled.
Another method has been proposed in Japanese patent publication 22503/1983 in which the flashing is carried out in two or more stages using two or more vessels kept under different reduced pressures.
However, in the method wherein the absorption liquid is recycled, the volatile organic compound will remain in the desorption column in an amount corresponding to the compound's vapor pressure under a reduced pressure at the stage of regeneration of the absorption liquid in the desorption column, and the remaining compound will obstruct the increase of the recovery percentage of volatile organic compound in the absorption column.